In the field of building construction, structural sheathing is a crucial element in suitable building design. Structural sheathing may serve many functions associated with the purpose and integrity of the assembly, including strengthening the building to lateral forces; providing a base wall to which finish siding can be nailed; acting as thermal insulation; and, in some cases, acting as a base for further thermal insulation. Structural sheathing, in the form of thin, rigid panels, is nailed directly onto the framework of the building. Some common types of sheathing include wood boards or slats, oriented strand board (OSB) panels, plywood panels, and gypsum panels.
Before the acceptance of performance-rated cellulose panels such as oriented strand board (OSB), plywood was the sheet product of choice for constructing wood shear walls. Plywood panels are very flexible and appropriate for a variety of building designs. The panel thickness, panel grade, nail type, and nail spacing could be combined in different ways to achieve a wall with the right design strength. In the 1970s, with the advent of performance-rated products based on waferboard technology, plywood was largely replaced with composite wood panels such as OSB. Today, all of the model building codes in the United States and Canada recognize OSB panels for the same uses as plywood on a thickness-by-thickness basis and they are used interchangeably, based on price and availability.
More recently, gypsum sheathing panels have been used as structural sheathing panels. Gypsum sheathing is most commonly manufactured with a water-resistive treated core but may also be available in a non-treated core. Treated core gypsum sheathing is intended for use as a substrate sheathing under a variety of exterior wall claddings in any climate. Non-treated core gypsum sheathing is intended for use only in dry climates. As with their wood counterparts, both types of gypsum sheathing (i.e. treated core and non-treated core) are designed to be mechanically attached to the outside surface of exterior wall framing using either nails, or screws, or staples. Gypsum sheathing is manufactured in a range of lengths and widths similar to those of both plywood and OSB.
The sheathing layer is designed with several system properties and requirements in mind. Of particular importance are the shear resistance imparted by the layer, the water vapor permeance of the layer, the weather resistance of the layer, and finally, the environmental impact (and associated greenhouse gases) involved with the manufacture of the sheathing layer. First, an appropriate structural building design requires that the panel reliably transfer shear forces (typically from wind shear or earthquake loads) from the body of the structure to its foundation. The performance of a panel in a building design is subject to many design elements including the material's Young's modulus, the panel thickness, the type and configuration of the structural framing and the type and spacing of the panel fasteners. All of these combine for a rated shear resistance in units of pounds per foot (lb/ft). Materials are tested via American Society of Testing and Materials (ASTM) test methods E2126 or E564. Further information may be found on the Internet at http://www.astm.org.
A second, important material property of the sheathing panel is the panel's role in the moisture management across the building envelope. The problems associated with excessive moisture in building wall cavities and the resulting mold growth, are well documented in the national outcry over unhealthy buildings and poor indoor air quality. As a result, building science has established best practices for minimizing the probability of mold growth in buildings. Walls between areas of differing temperature are the primary structures for these problems. Preventing condensation is of particular importance with regard to the exterior walls of homes or other buildings, where temperature extremes are likely to be greater than between interior walls. Wetting of exterior building surfaces and rainwater leaks are major causes of water infiltration, but so is excessive indoor moisture generation. Moisture may be present within a structure due to occupancy and use by humans, use of wet materials during construction, air leaks, or transfer from external wall materials.
A figure of merit for the measurement of the transport of water vapor across a component such as a panel or a wall assembly, is its permeance, or “perms”. One perm is defined as the transport of one grain of water per square foot of exposed area per hour with a vapor pressure differential of 1-inch of mercury (Hg). Vapor pressure is a function of the temperature and relative humidity (RH) of the air to which a test structure is exposed, and may be found in many standard data tables. The vapor pressure at any certain RH is found by the product of the RH and the vapor pressure for saturated air at a certain temperature. For example, at 70 degrees Fahrenheit the saturated vapor pressure is 0.7392 Hg and the vapor pressure at fifty per cent RH is 0.3696. The testing methodology varies depending upon the subject material. Data presented for sheathing panel products is typically taken using the ASTM E96 “dry cup” method. Further information may be found on the Internet at http://www.astm.org.
The Department of Energy (DOE) and the American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) and other building science organizations have established recommended wall designs that specify the proper location of a vapor retarder within the wall. A vapor retarder is defined by the building codes as a material having a permeance rating of 1.0 or less. Wall designs are dependant upon the regional climate of the building. In cooling-dominated climates, it is recommended that a vapor retarder be installed on the exterior of the thermal insulation—at the typical location of the structural sheathing. In hot and humid climates, such as along the Gulf coast and in Florida, the vapor barrier should be placed in this exterior location in the wall. Conversely, in heating dominated climates it is recommended that a vapor retarder be installed on the interior side of the thermal insulation—against the interior wallboard. In mixed zones, climates with both significant heating and cooling requirements, design recommendations suggest the omission of the vapor retarder altogether. If these guidelines are not observed, the structure is at risk of allowing water vapor condensation within the wall cavity and eventual water damage and mold growth. To avoid such an outcome, one must know the permeability of the wall components and use only appropriate materials.
For example, the rate of water vapor transmission of OSB is approximately two (2) perms. For sheathing grade plywood, of ½ to 1 inch thickness, the transmission rate is approximately ten (10) perms. Gypsum sheathing typically has an average vapor permeance of approximately twenty (20) perms. Therefore, plywood and gypsum are above the accepted minimum water vapor transmission rate of five (5) perms specified for a wall with a “U” value less than 0.25 and a vapor retarder not exceeding one (1) perm installed on the interior side of the framing. A double vapor retarder condition is avoided. However, OSB would be deemed unacceptable in the same assembly.
A third important material feature is weather resistance. Gypsum sheathing is designed for use as a substrate that is covered by an exterior wall cladding. Local weather conditions will dictate the length of time gypsum sheathing may be left exposed; however, it should perform satisfactorily if exposed to the elements for one month or less. Treated core gypsum sheathing should be covered immediately with a weather-resistive barrier, such as building felt or equivalent, if exposure time will exceed one month or weather conditions will be severe. Plywood can typically endure a similar period of weather exposure, while OSB can not. Non-treated core gypsum sheathing should be covered immediately after installation with a weather-resistive barrier. Gypsum sheathing does not hold peel and stick water barrier well.
Another final important consideration in the design and manufacture of construction materials is their potential negative environmental impact. Environmental impact can take many forms including the depletion of non-renewable natural resources (such as fossil fuels, for example), the generation of harmful chemicals or compounds, or the creation of greenhouse gasses. For a complete assessment as to the long term suitability of a construction material, the existing offering of sheathing materials should be considered in this context as well.
Unfortunately, the structural integrity of plywood is dependent upon the inclusion of quality wood laminates harvested from mature, large diameter trees, at least 30 years old. Their manufacture puts stress on old growth forests and existing woodland areas. As a result, much of the U.S. softwood plywood industry has shifted from the Pacific Northwest to the South and Southeast, to pine plantations on private lands. These small pines produce a lower quality panel than from the previously abundant older trees. In addition, their costs have risen over the last decade, making them less desirable as a mainstay construction material. OSB has at least two distinct advantages over traditional plywood panels. First, they do not require old growth forests, or decades old trees for their manufacture. OSB is derived from younger aspen trees of a much smaller relative diameter. Although the aspen wood is not a rapidly renewable resource, it does lessen the OSB's impact of endangered woodlands. Second, OSB extends the use of potentially dangerous resins such as phenol formaldehydes listed by the International Agency for Research on Cancer (IARC) as a potential carcinogen that may be released as an undesirable volatile organic compound (VOC) during the OSB's service life.
Gypsum sheathing panels do not require the use of wood and therefore do not share the concerns associated with tree harvesting. Instead, the manufacture of gypsum sheathing represents an astounding amount of embodied energy as a construction material. The term ‘embodied energy’ is defined as “the total energy required to produce a product from the raw materials stage through delivery” of finished product. Several of the steps (drying gypsum, calcining gypsum (dehumidification), mixing the slurry with hot water and drying the manufactured boards) involved in the manufacture of gypsum sheathing take considerable energy. Greenhouse gasses, particularly CO2, are produced from the burning of fossil fuels and the calcining of certain materials, such as gypsum. Thus the gypsum manufacturing process generates significant amounts of greenhouse gasses due to the requirements of the process.
According to the National Institute of Standards and Technology (NIST—US Department of Commerce), specifically NISTIR 6916, the manufacture of gypsum sheathing panel requires 8,196 British Thermal Units (BTU) per pound. With an average ⅝′ gypsum sheathing board weighing approximately 75 pounds, this equates to over 600,000 BTU's per board total embodied energy. Other sources suggest that embodied energy is less than 600,000 BTU's per board, while others suggest it may be even more. It has been estimated that embodied energy constitutes over 50% of the cost of manufacture. As energy costs increase, and if carbon taxes are enacted, the cost of manufacturing sheathing panel from calcined gypsum will continue to go up directly with the cost of energy. Moreover, material producers carry the responsibility to find less-energy dependent alternatives for widely used products as part of a global initiative to combat climate change.
For comparison, the same energy study (NISTIR 6916) found that a total of 18600 BTU's per panel are required for the wood harvesting and manufacture of plywood sheathing. OSB sheathing requires a similar amount of energy in its manufacture. Report NISTIR 6916 calculated 27100 BTU's per panel for OSB sheathing.
In summary, a product's potential negative environmental impact can take many forms, including a depletion of natural resources such as trees, potable water and materials in short supply, or the negative impact may be in the form of a significant consumption of energy during the product's manufacture and the resulting generation of greenhouse gases from its production.
Thus, it would be highly desirable to meet all of the performance requirements of a structural sheathing panel while reducing the environmental impact of its manufacture by reducing the harvesting of trees, reducing the use of harmful chemicals, and reducing the generation of dangerous greenhouse gases via a high embodied energy.